We propose to extend our x-ray absorption spectroscopic studies of active-site structures in the four known types of Ni-containing enzymes. Studies of the S-methyl coenzyme M reductase enzyme and its Ni-tetrapyrrole cofactor F 430 will concentrate on the Ni(I) oxidation state which is thought to be the active form. Newly synthesized Ni(I) forms of F 430 pentaalkylamides will be structurally characterized and their ligand-binding properties examined using XAS. Both plant (jackbean) and bacterial ( Klebsiella aerogenes ) ureases are thought to involve dinuclear Ni active sites and we will focus our attention on inhibited derivatives in which the Ni sites seem to be brought close enough to see Ni___Ni interactions in the Ni EXAFS.